wait... that means that the rotator is a surface element to the tank body (unless you are limiting turning sharpness, there isn't enough space for the tank to turn or amount of vehicle turns a game turn, would be useless (actually i retract that they are extremeley useful))

Basically, if a piece is said to be able to do something, like move or shoot ammunition, it's a surface element. If the piece more or less just sits there and does nothing but provide cover, it's a structural element.